The process of assembling electronic circuitry involves placing and soldering leaded and leadless components on a printed circuit board. During the assembly process, the components must be maintained in their proper positions prior to and during the soldering operation. In the case of leaded components, when the leads are inserted into holes in the printed circuit board they serve to aid in fixturing the component in place during the soldering operation. In both leaded and leadless component soldering, there exists a need to fixture the components so that the components do not move and remain in place.
This is accomplished by the use of an adhesive strategically placed underneath or along side the component. The adhesive is normally dispensed or deposited onto the printed circuit board and the components are placed in the adhesive and the adhesive is cured either thermally or with ultraviolet light. Solder is applied to the printed circuit board by stencilling or printing a solder paste, by cladding the printed circuit board with solder prior to assembly, or by wave soldering. In each of these cases, a solder flux must be provided in order to facilitate a sound metallurgical solder joint. Solder flux is normally formulated into the solder paste, applied as a coating over the entire circuit board when wave soldering, or is applied separately when solder clad circuit boards are used.
After soldering, the flux residue is removed from the board. This is typically accomplished by cleaning the soldered assembly in either an aqueous or organic cleaning solvent. The existence of an adhesive material underneath or adjacent to components provides a hindrance to cleaning and removing all traces of solder flux from beneath components. The standard industry practice to secure components in place is to use a thermosetting adhesive which cannot be removed after soldering. The cleaning operation is an expensive additional step and, in the case of organic solvents, utilizes material such as chlorofluorocarbons which have been deemed to be environmentally hazardous.
The prior art has attempted to utilize conventional soldering fluxes to hold component leads in place during soldering operations. U.S. Pat. No. 4,720,324 discloses the use of solder flux coated over the entire surface of the printed circuit board, and while in a tacky state, the components are added. The tack or stickiness of the flux is purported to hold the components in place. While it is clear that conventional soldering flux does provide some degree of tackiness, this method does not function adequately. The resulting coating is too thin to act as a sufficient tacking agent, and when small, closely spaced components are soldered, the components tend to be drawn to one another by surface tension and do not stay in place. Non-selective coating of the printed circuit board cannot be used with these small, light components, as they tend to swim about in the flux.
Clearly, there exists a need for a material that provides a temporary fixturing means for the components and also provides fluxing action for the soldering operation. In addition, such a material would ideally not need to be removed after the soldering operation, leaving no traces of solder flux residue or adhesive material surrounding the components.